Accurate measurement of current is required for different applications in the electricity consumption and generation fields. For example accurate current measurement is required for metering of electricity usage or generation, for over current protection and for sub-metering, e.g. in a building in which there are distinct electricity consumers for whom metering is required on an individual basis.
A current shunt provides one approach to measuring the high values of current encountered in such applications. In use a shunt of known resistance is provided in series with a load and the voltage developed across the shunt by the load drawn current is measured. The current passing through the shunt is then determined on the basis of Ohm's Law in view of the measured voltage and the known resistance of the shunt. Another approach to measuring high values of current involves the use of a current transformer wound on a core which is disposed around a conductor carrying current to be measured. The Hall current probe and the Rogowski coil provide further approaches to the measurement of high current. Each of these known approaches offers its advantages and disadvantages with one approach to current measurement being chosen in preference to the other approaches in dependence on requirements, e.g. with regards to accuracy, operating environment, space constraints, cost and the like.
The current shunt is capable of measuring both AC and DC and provides for linearity of measurement. Furthermore the current shunt is capable of providing absolute accuracy of measurement and temperature stability when properly calibrated and fabricated from a material having a very low temperature coefficient of resistance, such as manganin alloy. Certain applications, such as metering of electricity consumption and generation, require measurement to high accuracy over extended periods of time. For example in North America the ANSI C12.20 standard specifies an accuracy of ±0.5% for Class 0.5 consumption meters and ±0.2% for Class 0.2 consumption meters. Standards applicable in Europe and elsewhere, such as IEC 62053, specify similar accuracy requirements. Initial calibration to high accuracy is therefore normally required. The current shunt is, however, invasive and provides no isolation. The current transformer on the other hand provides for isolation and is less invasive but is capable of measuring AC only. In addition the current transformer is liable to non-linearity and phase error problems.
In contrast with the current transformer the Hall current probe is capable of measuring both AC and DC. In an open loop configuration the Hall current probe is, however, liable to non-linearity and temperature drift. When in a closed loop configuration the Hall current probe provides an improvement with regards to non-linearity and temperature drift although the weight and size of the configuration increases significantly where higher currents are measured. Turning to the Rogowski coil, this approach is entirely non-invasive because the coil is wound around a conductor which is to be the subject of measurement. The Rogowski coil offers the further advantage over the current transformer of being less liable to saturation because it lacks the iron core of the current transformer. However and as with the current transformer the Rogowski coil is capable of measuring alternating current only.
Ground fault conditions can present a risk of electric shock in electrical systems. Ground fault electric shock conditions can arise where there is insufficient grounding within an electrical system. For example the casing of electrical equipment may be improperly grounded such that when a person touches the casing he presents a lower impedance path to ground should the casing become live.
Ground fault electric shock conditions can also arise in electrical systems which meet accepted grounding practice. For example the TT grounding approach involves providing a ground at the utility pole and a ground directly to earth at each item of electrical apparatus. The TT grounding approach has been widely used in Europe mainly on account of the saving in wiring that the approach affords. Under certain circumstances the TT approach can, however, present problems. For example if a lightning surge on the power distribution lines produces a surge current of 1000 Amps which runs to earth at the utility pole, a voltage rise of 25,000 Volts is seen at the grounding electrode at the utility pole assuming the resistance between the grounding electrode and ground to be 25 Ohms. A resistance of 25 Ohms from the grounding electrode to earth meets NEC requirements. Where a first surface on electrical apparatus is connected to the utility pole ground and a second surface on the electrical apparatus is connected to a separate local ground direct to earth the 25,000 Volt signal appears across the first and second surfaces.
Ground fault electric shock conditions can arise even in an electrical system that is grounded according an approach, such as TN-C, which in contrast to the TT approach affords risk reduction in the face of lightning strikes and like fault conditions. More specifically and is almost universally appreciated a ground fault electric shock condition will arise when a person becomes the only path to ground for current flow by, for example, inserting a metal object into an electrical socket. No amount of grounding precautions will prevent electric shock in such circumstances.
Ground fault detectors are operative to determine if there is leakage of electrical current from an electrical circuit. Such leakage arises when there is a ground fault condition such as according to one of the examples given above. A ground fault detector may therefore provide a means to reduce the risk of electric shock. The ground fault detector operates on the basis that outwardly flowing current, e.g. in one or more live wires, must return, e.g. through a neutral wire, unless there is a current leakage path. It therefore follows that the sum of the currents flowing in conductors to and from an electrical load should be zero unless there is a leak. The differential current transformer is a known form of sensor which is operative to determine the sum of currents flowing in conductors to and from an electrical load. The differential current transformer comprises a core, which extends around the multiple conductors to be measured that form the primary and a multi-turn secondary winding, which is wound radially around the core. When the sum of the currents in the conductors passing through the core is zero no current signal is induced in the secondary winding. When the sum of the currents in the conductors passing through the core is more or less than zero a proportional current signal is induced in the secondary winding. The differential current transformer therefore provides a measure of the leakage current. A circuit breaker may then be operated in dependence on the secondary winding current exceeding a threshold value for a period of time, which corresponds to a maximum level of safe fault current. The response time of a leakage current detector can be in the range of 5 mS, 50 mS or 500 mS depending on the level of fault current. A circuit comprising a current sum sensor and a circuit breaker is termed a Ground Fault Circuit Interrupter (GFCI) in the US and a Residual Current Circuit Breaker (RCCB) or a Residual Current Device (RCD) amongst other terms in Europe. Sometimes the RCD term is used with respect to a device which is operative to detect leakage current but which lacks a circuit breaker.
An arc fault is another form of circuit condition that is liable to cause damage and be prejudicial to safety. An arc fault can generate high temperatures and thereby ignite combustible material. There are two forms of arc fault: the series arc fault; and the parallel arc fault. The series arc fault occurs across a discontinuity in a live or neutral conductor. Such a discontinuity is caused by, for example, a broken conductor, a loose terminal or a poor electrical connection at a wire nut. The current level in a series arc fault is limited by the impedance of the load. The parallel arc fault involves arcing between two conductors, such as between a live conductor and a neutral or grounded conductor, and typically arises when conductor insulation is damaged or deteriorates over time or through usage. The current level in a parallel arc fault is limited by the current available from the supply as limited by the impedance of the conductors carrying the fault current. Parallel arc faults therefore often involve higher levels of peak current than series arc faults. Furthermore the time constant that determines the length of time that an arc event is present is relatively short compared with other fault events. Typically the time constant is of the order of 10 nS, 100 nS, 1 uS or 10 uS depending on the line and load conditions. Therefore the peak current of the arc event may be present for insufficient time to trigger other fault detectors, such as over current or ground fault detectors. In addition arc fault determination often involves the analysis of multiple arc events for their periodicity and frequency.
The Arc Fault Circuit Interrupter (AFCI) is operative in the same fashion as the GFCI to open one or more ungrounded conductors when an unsafe circuit condition is detected. An arc fault is an intermittent condition which is characterised by a high peak current value but a low Root Mean Square (RMS) current value, which is generally below the normal operating threshold of a GFCI. In a first form an AFCI consequently comprises a current sensor, which is operative to measure the load current in one of the conductors, a waveform analysis circuit, which is operative on the output from the current sensor to discriminate between waveforms that are characteristic of normal circuit transients, such as transients caused by operation of wall switches, and waveforms that are characteristic of risk presenting arcs, and a circuit breaker that is operative in dependence on detection of an arc. This form of AFCI is capable of detecting and acting upon series and parallel arc faults. In a second form the AFCI comprises a differential current transformer disposed around the conductors to be monitored instead of the current sensor of the first form. A parallel arc fault from a conductor to ground produces a current on one of the conductors only, which is readily detected by the waveform analysis circuit. On the other hand a parallel arcing condition between the conductors produces equal and opposite currents in the conductors as in the GFCI as described above. However there is a phase difference between the current waveforms present in the two conductors. The differential transformer combined with a high pass filter allows the monitoring of any high frequency transient without need to handle the dynamic range on the lower frequency normal waveform. The differential transformer rejects any common signal such as the mains load current while passing any difference as may be caused by the time delay difference between live and neutral that will typically occur in a series arc fault, to thereby effectively act as a high pass filter and improve the dynamic range requirement to extract an arc event. The current transformer is sometimes combined with an extra high pass or band pass filter to further select the characteristics of interest for only arc fault detection. The waveform analysis circuit of the AFCI is therefore operative at a sufficiently high frequency to identify and act upon the current waveform present in at least one of the conductors to thereby detect the arc fault.
The most familiar application of electricity measurement is in electricity consumption metering for invoicing purposes. With the development of local electricity generation capabilities metering of generated electricity for invoicing purposes is becoming more widely used. Beyond invoicing, electricity metering sees application in demand monitoring which is of importance to the electricity generator and distributor for determining usage patterns and trends. Electricity metering is also seeing increased use in the smart grid as a means to determine the behaviour and actions of suppliers and consumers connected to the grid. As mentioned above certain applications of electricity metering require measurement to high accuracy over extended periods of time with the ANSI C12.20 standard in North America specifying an accuracy of ±0.5% for Class 0.5 consumption meters and ±0.2% for Class 0.2 consumption meters. Standards applicable in Europe and elsewhere specify similar accuracy requirements.
Digital electricity meters have been used for some years. Such digital electricity meters typically comprise a potential divider for measurement of voltage. There are different approaches to current measurement depending on circumstances and requirements. Normally a current sensor is provided on the live conductor only. In some cases, however, there is a current sensor on each of live and neutral for the purpose of crude tamper detection. Although such tamper detect arrangements sometimes take account of the measurements on live and neutral they do not do so to any degree of accuracy or provide for fault detection. One approach involves the use of a shunt resistor in the live conductor and a current transformer on the neutral conductor. Another approach involves the use of a current transformer on each of the live and neutral conductors. A further approach involves the use of a shunt resistor on each of the live and neutral conductors with the electricity meter being configured to maintain isolation between the live and neutral conductors despite the galvanic connection to both live and neutral conductors.
Sub-metering provides for billing of individual consumers where the electricity utility is unable or unwilling to measure the consumption of such individual consumers. Typical users of sub-metering include apartment complexes, commercial buildings and mobile home parks. Individual metering of electricity consumption has the advantage of creating awareness of energy conservation on the part of the consumer. Alternatively sub-metering can take place at the point of load, i.e. at the electrical apparatus. Sub-metering at the point of load can provide an indication of improper operation of the electrical apparatus, e.g. as reflected by an unusual increase in consumption. Furthermore sub-metering at the point of load provides the consumer with insight as to the extent of consumption of the electrical apparatus in absolute and comparative terms.
Fault detection, such as by the GFCI and the AFCI, has seen increased use over the years as a means to improve upon personal safety and to reduce the incidence of damage to property through fires. The design and operation of fault detectors is subject to standards created by various bodies. For example the National Electrical Manufacturers Association (NEMA), which represents the interests of electro-industry product manufacturers in the US, publishes standards relating to fault detectors primarily for the US market. The Underwriters Laboratories (UL) also publishes standards for electrical safety equipment. A further example is the International Electrotechnical Commission (IEC) which has been the primary organisation for creating standards, which although international in scope are biased towards European practices. Legislation and regulations in certain jurisdictions has been a primary motivator for increased use of fault detectors. For example Germany has required the use of Residual Current Devices (RCDs) on sockets up to 20 Amps from June 2007, Norway has required the use of RCDs in all new homes since 2002 and all new sockets since 2006 and the UK has required RCDs in all new installations since 2008. For ground fault devices there is an IEC specification, namely IEC 61008 and a UL specification, namely UL 943. AFCI's have been of greater interest in the US and Canada in part on account of the prevalence of wooden and hence fire damage prone buildings. In a pan European context, the MID (Measuring Instruments Directive) took effect on 30 Oct. 2006 with the aim of creating a single market for measuring instruments across the European Union. The objects of the MID are to guarantee a high level of safety and reliability for certified measuring instruments and provide for protection against data corruption in such measuring instruments whilst providing for free circulation of measuring instruments within the European Union. Annexes to the MID define how measuring instruments can be certified as compliant. Notified bodies are authorised to carry out testing of measuring instruments, with certificates issued by a notified body being accepted throughout the European Union. The MID supersedes national measures such as the OFGEM approval process in the UK. As further examples of national measures, the 1999 version of the National Electrical Code (NFPA 70) in the United States (US) and the 2002 version of the Canadian Electrical Code (CSA Standard C22.1) each require AFCI's in all circuits feeding outlets in bedrooms of dwellings. AFCI's are subject to a UL specification, namely UL1699. A more recent example is the 2008 National Electrical Code requirement for installation of combination-type AFCI's in all 15 and 20 Ampere residential circuits with the exception of laundries, kitchens, bathrooms, garages and unfurnished basements.
The design and operation of fault detectors is less than straightforward compared, for example, to the design and operation of over-current detectors. More specifically different forms of fault detector, such as the GFCI and the AFCI, involve different electrical designs. Indeed requirements may differ within a particular class of fault detector. More specifically a parallel arc fault typically manifests as an intermittent current in excess of 75 Amps whereas a series arc fault manifests as an intermittent current in excess of 5 Amps. Furthermore the maximum level of peak current depends on the time constant and the form of electrical circuit in which the device is used. Fault characteristics may be difficult to distinguish from the current consumption characteristics of equipment normally attached to the same electrical network. For example the initial current drawn by a motor may appear like an arc fault and this may lead to false tripping. Another consideration is the differing requirements from jurisdiction to jurisdiction. For example Class A GFCIs have a minimum must trip value of between 4 mA and 6 mA in the US whereas the RCD, which is the equivalent device in Europe, has a trip value of 30 mA. Furthermore the required time to trip often depends on the level of fault current with a higher level of fault current requiring a shorter time to trip. Improper operation, such as on account of false triggering, over sensitivity or under sensitivity, provides for further complication.
In addition requirements differ from electrical installation to electrical installation. For example one installation may require GFCI and over-current protection whereas another installation may require GFCI, AFCI and over-current protection along with a current measuring capability. Such differing requirements are met by installing plural devices. Where multiple functionality is required in the deployment of switchgear in a building multiple different devices are connected in series. This is likely to present an issue of cost and size or involve limiting capabilities by sharing components between or amongst plural systems. For example an RCD or sub-meter may be shared amongst several circuit breakers.
The present invention has been devised in the light of the inventors' appreciation of the above mentioned problems. It is therefore an object for the present invention to provide improved current measurement apparatus configured to measure current in a live conductor and a neutral conductor. It is a further object for the present invention to provide an improved method of measuring current comprising measuring current in a live conductor and a neutral conductor.